The invention relates to sacrificial energy-absorbing structures constructed and intended to deform permanently in absorbing energy. Such structures find use in crash protection for automobiles and other vehicles to protect the occupants of the vehicles or other road users.
It is known to provide such deformable structures in the form of an array of honeycomb-shaped cells, the walls of which are made from a material such as metal foil, and which array is progressively crushed to absorb kinetic energy, e.g. in an automobile accident. Such honeycomb structures are effective as crash protection for automobiles and the like but are difficult and thus expensive to make. Therefore, such structures are unlikely to be widely adopted in cheaper motor vehicles. One such structure is described in WO98/06553 (Cellbond Composites Ltd and Bayerishe Motoren Werke AG).
A structure is described in GB 1 225 681 as a core material for a lightweight structure. The structure is intended to have a maximum strength for lowest cost, and there is no mention of energy absorption. Indeed, the drawings show sidewalls that are too close to perpendicular to the plane of the sheet for good energy absorption by plastic deformation. Another prior art document, GB 1 305 489, describes a packaging material for use in containers to prevent breakage. That document describes the use of microcellular expanded polystyrene. Such a material would be far too soft to absorb any significant amount of energy in a vehicle crash, and hence would not avoid injury. A further document, GB 1 420 929, describes a xe2x80x9ccuspatedxe2x80x9d sheet material that is said to be suitable for a variety of uses. The material has narrow xe2x80x9ccuspsxe2x80x9d and the sidewalls of the xe2x80x9ccuspsxe2x80x9d are too close to the direction perpendicular to the sheet to plastically deform properly to absorb impact.
According to the invention there is provided an energy-absorbing vehicle component for reducing the risk of injuries, wherein the component comprises a sheet extending substantially in a median plane and formed to have a pattern of alternating front and rear projections in front of and behind the median plane. The projections are arranged to alternate in two directions in the median plane. The sheet is at an angle of 25 to 60 degrees from the median plane of the sheet at the position of maximum steepness on the line between a front projection and an adjacent rear projection. The component absorbs impact by plastic deformation of the sheet such that the curve of stress normal to the median plane to deform the sheet against the deformation exhibits a plateau at a predetermined level.
The angle is shown as xcex8 on FIG. 2. The maximum steepness preferably occurs midway between the adjacent front and rear projections. Indeed, the sheet is preferably smooth from one front peak to an adjacent rear peak.
It is not necessary that the median plane be flat. Rather, it is the notional plane that locally represents the position of the sheet with the projections smoothed out. If in addition to the projections the component is macroscopically curved over its surface, for example to fit the interior of a motor vehicle, then the median plane will not be flat over the whole surface. The projections extend in front of and behind this notional median plane.
The pattern of projections need not have the same number of front projection adjacent to every rear projection, nor must the front and rear projections necessarily have the same shape. Indeed, it is possible that the front or rear projections are joined together in groups; this can occur if only front projections are formed in a flat sheet. The portion of the sheet that is unformed then constitutes linked rear projections behind the notional median plane.
By a plateau is meant a substantially flat region of the curve, in which a percentage change in deformation is accompanied by a much smaller percentage change in applied stress. By xe2x80x9cmuch smallerxe2x80x9d is meant that the percentage change of applied stress is at most 15% of the percentage change in deformation, preferably at most 10% or further preferably at most 5%.
An alternative definition of plateau, also intended, is that the curve does not deviate by more than 10%, preferably 5%, from the predetermined level.
The flat region of the curve preferably extends from a deformation of at most 70%, further preferably at most 50% or even 35% of the deformation at the maximum deformation of the plateau up to that maximum plateau deformation. Accordingly, the plateau can cover 65% of the range of deformation available.
The predetermined plateau level may be a level of stress that does not cause serious injury or that avoids death when a human body part such as a head impacts the energy-absorbing sheet. The level may be in the range 0.2 to 0.3 MPa. The level may be arrived at by determining the material of the sheet, the thickness of the sheet, the pitch density of projections over the area of the sheet, and the shape of the projection.
The projections may cover at least 80% of the area of the sheet leaving no substantial areas therebetween.
The most commonly used energy absorber at present is a rigid polyurethane foam. In tests, one of which is presented later, the deformation of such a foam does not show a plateau in the same way as the formed sheet energy absorber according to the invention.
The human body, especially the human head, can take certain levels of stress before injury or death. These levels are specified by car manufacturers when selecting energy absorbing materials for use in their vehicles. A plateau on the stress-deformation graph allows greater energy absorption without injury or death than a more proportional stress-deformation graph as seen in conventional foam.
The invention provides an energy absorbing material that can be manufactured much more cheaply than the honeycomb structure but can still provide good protection.
However, in spite of some similarities of structure, none of the prior documents discussed above suggests an energy-absorbing structure as described above; rather, they disclose cell structures for lightweight, strong materials. Plastic deformation to provide sacrificial energy absorption, would not be wanted in any such structure. None of the documents teaches forming a plateau in the stress-deformation curve in the manner of the structure according to the invention. The wall thickness of the projections may be no greater than that of the thickness of the undeformed sheet. The structure may be a composite in which the sheet forms a core and has a face sheet or skin applied to one or both sides. The face sheet(s) or skin(s) may be planar. A skin may be especially useful if the projections are large; this may be the case when thicker aluminium panels are used. Preferably the skins are attached in such a way that some lateral movement (in the plane of the component) is possible during impact. It has been found that sliding of the front and rear faces of the sheet against the skins during impact improves the impact-absorbing properties of the material.
The method of fixing the skin to the sheet may be to fix some (for example up to 20%) of the projections to the skin. Indeed, the skin can conveniently be fixed using just four projections. This leaves the remainder of the projections free to slide. The fixing may be carried out using welding, e.g. ultrasonic welding, rivets or clinching.
If all the peaks are to be bonded then it is preferred to use a weaker fixing method such as polyurethane adhesive which still allows movement during impact.
The material of the sheet may be aluminium, which is very lightweight. Alternatively, thermoplastics may be moulded into a suitable shape.
The invention also envisages a vehicle comprising an energy-absorbing vehicle component as defined above. The component may be mounted behind the headlining of the vehicle to absorb impacts of the head of an occupant against the headlining during a crash. Alternatively, the material is sufficiently cheap and easy to form that the component may be mounted on the rear surfaces of seats in cars, aircraft or other vehicles to absorb the impact of the head or knees of an occupant sitting immediately behind the seat.
The component may be mounted behind a fabric sheet or other means to hide the component from view. The component can be mounted behind the headlining of a car.
A method of making an energy-absorbing component comprises providing a blank of sheet material and acting on the blank to deform the blank into a three-dimensional form. The deforming step may comprise a pressing operation. The deforming step may be such as to form discrete projections extending from the plane of the sheet and of the desired shape.
Thus the projections may, for example, be domed, pyramidal or the like. The projections may be frusto-conical. The projections may all be the same or may be different, e.g. in height and/or in shape as desired.
In some cases, the projections may be elongate, and may in the extreme case be corrugations extending the whole width of the sample. However, it is more difficult to get corrugations to give a good plateau than it is using preferred projections which are roughly as long as they are wide.
A solid lubricant may be used between the mould and the sheet to make pressing easier. The solid lubricant can be thin plastic sheet.